Update dependencies and add support for Null Safety

[Yi-Jiahe]

Dropped my_complex dependency in favor of more popular and updated complex package with null support and migrate to support null safety.

Update tuple to 2.0.0.

Removed benchmarking tests for my_complex adding multiplication, turning and invert.

[Yi-Jiahe]

Incidentally, I modified the benchmarks to work with the complex package:

 test("complexpolar adding timed", () {
    Stopwatch s = new Stopwatch();
    var cp4 = Complex.polar(1, 1);
    var cp3 = Complex.polar(1, 0);
    num reps = math.pow(10, 6);
    s.start();
    for (int i = 0; i < reps; i++) {
      cp3 = cp3 + cp4;
    }
    s.stop();
    print("$reps repetitions: ${s.elapsedMilliseconds}ms");
  });

  test("complexpolar multiplication timed", () {
    Stopwatch s = new Stopwatch();
    var cp4 = Complex.polar(1, 1);
    var cp3 = Complex.polar(1, 0);
    num reps = math.pow(10, 6);
    s.start();
    for (int i = 0; i < reps; i++) {
      cp3 = cp3 * cp4;
    }
    s.stop();
    print("$reps repetitions: ${s.elapsedMilliseconds}ms");
  });

  test("complexpolar imaginary timed", () {
    Stopwatch s = new Stopwatch();
    var cp4 = Complex.polar(1, 1);
    double total = 0.0;
    num reps = math.pow(10, 6);
    s.start();
    for (int i = 0; i < reps; i++) {
      total += cp4.imaginary;
      cp4 = cp4 * Complex.polar(0.1);
    }
    s.stop();
    print("$reps repetitions: ${s.elapsedMilliseconds}ms");
  });

  test("complexpolar invert timed", () {
    Stopwatch s = new Stopwatch();
    var cp4 = new Complex.polar(1, 1);
    num reps = math.pow(10, 6);
    s.start();
    for (int i = 0; i < reps; i++) {
      cp4 = Complex(1) / cp4;
    }
    s.stop();
    print("$reps repetitions: ${s.elapsedMilliseconds}ms");
  });

And the results were
Original:

$ dart test
Building package executable...
Built test:test.
00:00 +3: test\fft_test.dart: complexpolar adding timed
1000000 repetitions: 29ms
00:00 +6: test\fft_test.dart: complexpolar multiplication timed
1000000 repetitions: 18ms
00:01 +7: test\fft_test.dart: complexpolar imaginary timed
1000000 repetitions: 24ms
00:01 +8: test\fft_test.dart: complexpolar invert timed
1000000 repetitions: 19ms
00:01 +17: All tests passed!

Updated:

$ dart test
00:00 +0: test\fft_test.dart: complexpolar adding timed
1000000 repetitions: 16ms
00:00 +4: test\fft_test.dart: complexpolar multiplication timed
1000000 repetitions: 13ms
00:00 +7: test\fft_test.dart: complexpolar imaginary timed
1000000 repetitions: 19ms
00:00 +8: test\fft_test.dart: complexpolar invert timed
1000000 repetitions: 19ms
00:01 +17: All tests passed!